1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to coherent detectors for optical receivers.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A coherent optical-detection scheme is capable of detecting not only the amplitude of an optical signal, but also the signal's polarization and phase. These capabilities make coherent optical detection compatible with polarization-division multiplexing and with the use of spectrally efficient modulation formats, such as quadrature amplitude modulation (QAM) and phase-shift keying (PSK) in its various forms (e.g., differential PSK (DPSK) and differential quadrature PSK (DQPSK)). Compared to incoherent detectors, coherent optical detectors offer relatively easy wavelength tunability, good rejection of interference from adjacent channels in dense wavelength-division-multiplexing (DWDM) systems, linear transformation of the electromagnetic field into an electrical signal for effective application of modern digital signal processing techniques, and an opportunity to use polarization-division multiplexing. As a result, coherent optical detectors are currently being actively developed.